fix: write remote completion logs as utf-8

[he-yufeng]

Summary

• write RemoteConversation completion logs with explicit UTF-8 encoding on the client
• add a regression test for non-ASCII completion log data

Addresses OpenHands/OpenHands#14389.

To verify

• uv run pytest tests\sdk\conversation\remote\test_remote_conversation.py -q --basetemp C:\dev\GITHUB-clean_tmp\software-agent-sdk-pytest
• uv run ruff check openhands-sdk\openhands\sdk\conversation\impl\remote_conversation.py tests\sdk\conversation\remote\test_remote_conversation.py
• uv run ruff format --check openhands-sdk\openhands\sdk\conversation\impl\remote_conversation.py tests\sdk\conversation\remote\test_remote_conversation.py
• uv run pyright openhands-sdk\openhands\sdk\conversation\impl\remote_conversation.py tests\sdk\conversation\remote\test_remote_conversation.py
• python -m py_compile openhands-sdk\openhands\sdk\conversation\impl\remote_conversation.py tests\sdk\conversation\remote\test_remote_conversation.py
• git diff --check

[he-yufeng]

Rebased onto current main and resolved the test conflict by keeping both the upstream ACP endpoint coverage and this PR's UTF-8 completion-log regression test.

Validated locally:

• .venv\Scripts\python.exe -m pytest tests\sdk\conversation\remote\test_remote_conversation.py -q --basetemp C:\dev\GITHUB-clean\_tmp\software-agent-sdk-pytest
• .venv\Scripts\python.exe -m py_compile openhands-sdk\openhands\sdk\conversation\impl\remote_conversation.py tests\sdk\conversation\remote\test_remote_conversation.py
• .venv\Scripts\python.exe -m ruff check openhands-sdk\openhands\sdk\conversation\impl\remote_conversation.py tests\sdk\conversation\remote\test_remote_conversation.py
• git diff --check